boundary

That which delineates a particular space from another space or the rest of the world. Broadly speaking, the separation between established Space and changes of Time.

Example:
The walls of a home protect what is inside from the dangerous elements of nature.
The skin of a human protects and contains the soft internal organs from foreign material and the drying, burning light of the sun.
The identity, customs. and rules of a society separate it from the foreign elements which may undermine or alter it.